β-Oxidation and Omega-Oxidation of Saturated Fatty Acids
Interactive E-Content Module
Prepared Following UGC Four Quadrant Approach
Author: Dr. Chandralekha Deka, Assistant Professor, Department of Zoology, PDUAM, Amjonga
QUADRANT I — E-TUTORIAL
Introduction
Fatty acids are an important source of energy in living organisms. During starvation, prolonged exercise, or carbohydrate deficiency, fats become the major energy source. Fatty acids are broken down mainly through β-oxidation and omega-oxidation pathways.
β-oxidation occurs primarily in mitochondria and produces acetyl-CoA, NADH, and FADH₂. Omega-oxidation occurs in the endoplasmic reticulum and serves as an alternative pathway.
Learning Objectives
- Define β-oxidation and omega-oxidation.
- Explain activation and transport of fatty acids.
- Describe β-oxidation of even-chain fatty acids.
- Explain oxidation of odd-chain fatty acids.
- Differentiate β-oxidation and omega-oxidation.
- Discuss biological significance of fatty acid oxidation.
Fatty Acid Oxidation
Fatty acid oxidation is the catabolic process in which fatty acids are broken down to release energy.
Major Pathways
- β-oxidation
- Omega-oxidation
β-Oxidation of Fatty Acids
Definition
β-oxidation is the process by which fatty acids are degraded in mitochondria through successive removal of two-carbon units in the form of acetyl-CoA.
The name β-oxidation is given because oxidation occurs at the β-carbon atom of the fatty acid.
Site of β-Oxidation
- Mainly occurs in mitochondrial matrix.
- Very long-chain fatty acids may initially undergo oxidation in peroxisomes.
Activation of Fatty Acids
Before oxidation, fatty acids are activated in the cytoplasm.
Reaction
Enzyme
Acyl-CoA synthetase (thiokinase)
Transport of Fatty Acids into Mitochondria
Long-chain fatty acyl-CoA cannot directly enter mitochondria.
Carnitine Shuttle
Transport occurs with the help of carnitine.
Steps
- Formation of acyl-carnitine.
- Transport across inner mitochondrial membrane.
- Regeneration of fatty acyl-CoA inside matrix.
Steps of β-Oxidation
Step 1: Oxidation
Enzyme: Acyl-CoA dehydrogenase
Fatty acyl-CoA is converted into trans-enoyl-CoA.
Product: FADH₂
Step 2: Hydration
Enzyme: Enoyl-CoA hydratase
Water is added across the double bond.
Product: β-hydroxyacyl-CoA
Step 3: Second Oxidation
Enzyme: β-hydroxyacyl-CoA dehydrogenase
β-hydroxyacyl-CoA is oxidized.
Product: NADH
Step 4: Thiolysis
Enzyme: Thiolase
β-ketoacyl-CoA is cleaved by CoA.
Products:
- Acetyl-CoA
- Fatty acyl-CoA shortened by two carbon atoms
β-Oxidation of Saturated Fatty Acids with Even Number of Carbon Atoms
Example: Palmitic Acid (16 Carbon Atoms)
Palmitic acid undergoes seven cycles of β-oxidation.
Final Products
| Product | Number |
|---|---|
| Acetyl-CoA | 8 |
| NADH | 7 |
| FADH₂ | 7 |
β-Oxidation of Saturated Fatty Acids with Odd Number of Carbon Atoms
Example: Pentadecanoic Acid (15 Carbon Atoms)
Odd-chain fatty acids undergo β-oxidation similarly until the final cycle.
Final Products
- Acetyl-CoA
- Propionyl-CoA (3-carbon compound)
Conversion of Propionyl-CoA to Succinyl-CoA
Step 1
Propionyl-CoA → D-methylmalonyl-CoA
Enzyme: Propionyl-CoA carboxylase
Cofactor: Biotin
Step 2
D-methylmalonyl-CoA → L-methylmalonyl-CoA
Enzyme: Racemase
Step 3
L-methylmalonyl-CoA → Succinyl-CoA
Enzyme: Methylmalonyl-CoA mutase
Cofactor: Vitamin B₁₂
Importance of Succinyl-CoA
- Enters Krebs cycle.
- Can contribute to gluconeogenesis.
Thus, odd-chain fatty acids are partially glucogenic.
Omega (ω)-Oxidation of Fatty Acids
Definition
Omega-oxidation is an alternative pathway of fatty acid oxidation in which oxidation occurs at the terminal methyl carbon (ω-carbon).
Site of Omega-Oxidation
- Smooth endoplasmic reticulum
- Mainly in liver and kidney
Steps of Omega-Oxidation
Step 1: Hydroxylation
The terminal methyl group is converted into alcohol.
Enzyme: Mixed function oxidase
Step 2: Oxidation to Aldehyde
Alcohol is converted into aldehyde.
Step 3: Oxidation to Carboxylic Acid
Aldehyde is converted into dicarboxylic acid.
The dicarboxylic acid may further undergo β-oxidation.
Biological Significance of Omega-Oxidation
- Alternative pathway during defective β-oxidation.
- Helps detoxification of fatty acids.
- Important in metabolism of medium-chain fatty acids.
Difference Between β-Oxidation and Omega-Oxidation
| Feature | β-Oxidation | Omega-Oxidation |
|---|---|---|
| Site | Mitochondria | Endoplasmic reticulum |
| Carbon oxidized | β-carbon | ω-carbon |
| Major pathway | Yes | Minor pathway |
| Main products | Acetyl-CoA | Dicarboxylic acids |
| Importance | Energy production | Alternative pathway |
Clinical Importance
Defects in fatty acid oxidation may lead to:
- Hypoglycemia
- Muscle weakness
- Neurological disorders
Vitamin B₁₂ deficiency affects conversion of methylmalonyl-CoA to succinyl-CoA.
Conclusion
β-oxidation is the major pathway of fatty acid catabolism and provides a large amount of energy through production of acetyl-CoA, NADH, and FADH₂. Even-chain fatty acids produce only acetyl-CoA, whereas odd-chain fatty acids additionally produce propionyl-CoA, which is converted into succinyl-CoA. Omega-oxidation acts as an alternative pathway and forms dicarboxylic acids.
QUADRANT III — SELF-ASSESSMENT
MCQs
- β-oxidation occurs in:
- a) Nucleus
- b) Mitochondria
- c) Ribosome
- d) Golgi body
- The transport of fatty acids into mitochondria requires:
- a) Biotin
- b) Carnitine
- c) ATP synthase
- d) Hemoglobin
- The final product of odd-chain fatty acid oxidation is:
- a) Pyruvate
- b) Oxaloacetate
- c) Propionyl-CoA
- d) Citrate
References
- Lehninger A.L. Principles of Biochemistry.
- Nelson D.L. and Cox M.M. Lehninger Principles of Biochemistry.
- Voet D. and Voet J. Biochemistry.
- Stryer L. Biochemistry.
- Satyanarayana U. Biochemistry.
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